Exciton Spin Dynamics in Semiconductor Quantum Dots
Semiconductor quantum dots are nanometer sized objects that contain typically several thousand atoms of a semiconducting compound resulting in a confinement of the carriers in the three spatial directions. They can be synthesized by a large variety of met
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4.1 Introduction Semiconductor quantum dots are nanometer sized objects that contain typically several thousand atoms of a semiconducting compound resulting in a confinement of the carriers in the three spatial directions. They can be synthesized by a large variety of methods based on colloidal chemistry [1, 2], molecular beam epitaxy or metalorganic chemical vapor deposition. Quantum dots can be formed at interface steps of thin quantum wells [3, 4] or by self-assembly in the Stransky–Krastanov growth mode during molecular beam epitaxy. This process is driven by the strain resulting from the difference in lattice parameter between the matrix (barrier) and the dots, for example 7% for InAs dots in GaAs. The quantum dots obtained in this well-studied system are typically 20 nm in diameter and 5 nm in height and are formed on a thin quantum well referred to as a wetting layer, see Fig. 4.1 for a transmission electron microscope image [5]. Samples used for optical spectroscopy are then covered again by the barrier material. The Stransky–Krastanov growth mode is applied to a large variety of III–V and II–VI compounds [6–8]. An interesting alternative for fabricating GaAs or InAs dots is provided by a technique which is not strain driven, called molecular droplet epitaxy [9]. Quantum dots defined by electrostatic potentials have also shown very interesting effects at very low temperature [10, 11]. Quantum dots are often referred to as artificial atoms because of the discrete nature of their valence and conduction states. The discrete nature of the electronic levels involved in the optical transitions lies at the origin of fascinating experiments that use dots as emitters of single, indistinguishable, and entangled photons [12–14]. Long optical coherence times [15] have made coherent control experiments possible [7, 16]. In this chapter we describe the carrier spin dynamics in quantum dots with a direct band gap. The absence of translational motion prolongs the carrier spin lifetimes as compared to bulk (3D) and quantum well (2D) structures, because spin relaxation mechanisms based on the spin–orbit interaction are strongly inhibited—see Chap. 1.
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Fig. 4.1. (a) Electron microscopy image of an InAs dot in GaAs [5]. (b) Sketch of the lowest lying, discrete energy states in a quantum dot, where the energy separation Eg is determined by the band gap energy and strain in the semiconducting quantum dot material
However, two spin interactions are enhanced by the strong quantum confinement, namely the Coulomb exchange interaction between carriers and the hyperfine interaction between electron and nuclear spins [17]. The latter is a fascinating subject in its own right and can give rise, for example, to dynamical nuclear polarization [18, 19], electron spin dephasing [10, 11, 20, 21] and bistable nuclear spin configurations [22, 23]. The hyperfine effects will be evoked in this chapter where appropriate, but for a more detailed discussion the reader is referred to Chap. 11. In the following sections we will focus
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